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WriteUp来源

来自M01N战队

题目描述

某安全机构截获了一段密文，并通过社工的方式获取了该密文对应的加密方式。现请你帮助他们对该密文进行解密。4441774e565535504f535a4e524870556446414f5a45496b6152634f41424e49483355774e5339414d44553d

题目考点

• 算法分析

解题思路

根据题意编写解密算法，得到flag

import struct
import base64
import binascii
flag = 'xxxxxxxxxxxxxxxxxxxxx'
key = 0x326c3567

def crypto(data):
    #print(data,data>>16)
    return data ^ data >> 16

def encode(data_content, key):
    cyphertext = []
    data_content_length = len(data_content)
    cyphertext += [crypto(data_content[0] ^ key)]

    for i in range(1, data_content_length):
        cyphertext += [crypto(cyphertext[i-1] ^ data_content[i])]

    crypted_result = ''
    for c in cyphertext:
        crypted_result += struct.pack("I", c)

    return base64.b64encode(crypted_result)
def decode(p_en_content,key):

    p_en_content=base64.b64decode(p_en_content)
    en_content = struct.unpack("I" * (len(p_en_content) / 4), p_en_content)
    print(en_content)
    plain_text=[]
    en_len= len(en_content)
    plain_text+=[crypto(en_content[0])^key]
    for i in range(1, en_len):
        plain_text += [crypto(en_content[i])^en_content[i-1] ]
    decrypted_result = ''
    for c in plain_text:
        decrypted_result += struct.pack("I", c)

    return decrypted_result

padding = 4 - len(flag) % 4
if padding != 0:
    flag = flag + "\x00" * padding

data_content = struct.unpack("I" * (len(flag) / 4), flag)
en_data=encode(data_content, key)
print(decode(en_data,key))
#data_content = struct.unpack("I" * (len(flag) / 4), flag)
en_flag=binascii.a2b_hex('4441774e565535504f535a4e524870556446414f5a45496b6152634f41424e49483355774e5339414d44553d')
print(decode(en_flag,key))
print('%x'%crypto(crypto(0x45678922)))

Flag

flag{e4sy_Cr7pt0_cgs_lz_!@#}